JP2651625B2 - Energy subtraction image generation method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an energy subtraction image generation method for reducing noise in an energy subtraction image of a radiation image and obtaining an image with excellent observation performance.

(Prior art) Reading a recorded radiation image to obtain image data,
After subjecting the image data to appropriate image processing, an image is reproduced and recorded in various fields. For example, an X-ray image is recorded using an X-ray film having a low gamma value designed to be compatible with later image processing, and this X-ray image is recorded.
The X-ray image is read from the film on which the line image is recorded, converted into an electric signal, the electric signal (image data) is subjected to image processing, and then reproduced as a visible image in a copy photograph or the like, so that the contrast and sharpness are improved. A system capable of obtaining a reproduced image having good image quality performance such as graininess has been developed (see Japanese Patent Publication No. 61-5193).

In addition, the applicant has determined that radiation (X-ray, α-ray, β-ray,
Ray, electron beam, ultraviolet ray, etc.), a part of this radiation energy is accumulated, and then, when irradiated with excitation light such as visible light, stimulable fluorescent light that emits the amount of photostimulated light corresponding to the accumulated energy Using the body (stimulable phosphor)
A radiation image of a subject such as a human body is once photographed and recorded on a sheet-shaped stimulable phosphor, and the stimulable phosphor sheet is scanned with excitation light such as a laser beam to generate stimulable emission light. A radiation recording / reproducing system that photoelectrically reads emitted light to obtain an image signal and outputs a radiation image of a subject as a visible image to a recording material such as a photographic photosensitive material or a CRT based on the image signal has already been proposed. (Japanese Patent Laid-Open No. 55-12429)
Nos. 56-11395, 55-0163472, 56-164645
No. 55-116340, etc.).

This system has the practical advantage of being able to record images over a very wide radiation exposure area compared to conventional silver halide photography radiography systems. That is, it has been recognized that the amount of stimulating light emitted by excitation after accumulation with respect to the amount of radiation exposure is proportional over a very wide range, and thus the amount of radiation exposure varies considerably depending on various imaging conditions. However, the stimulated emission light emitted from the stimulable phosphor sheet is read, the gain is set to an appropriate value, read by photoelectric conversion means, converted into an electric signal (image data), and the photographic light is By outputting a radiation image as a visible image to a display device such as a material or a CRT, a radiation image that is not affected by a change in radiation exposure amount can be obtained.

In a system using an X-ray film or a stimulable phosphor sheet as described above, a plurality of recorded radiation images are read to obtain a plurality of image data, and the subtraction of the radiation image is performed based on these image data. Processing may be applied.

Here, the subtraction processing of the radiation image refers to a processing of obtaining an image corresponding to a difference between a plurality of radiation images captured under different conditions, and specifically, the plurality of radiation images are obtained at a predetermined sampling interval. By reading, a plurality of digital image signals corresponding to each radiation image are obtained, and a subtraction process is performed for each corresponding sampling point of the plurality of digital image signals to emphasize only a specific subject portion in the radiation image. Or, a process of obtaining an extracted radiation image.

This subtraction processing basically includes the following two. That is, the subtraction (subtract) of a radiographic image in which a contrast agent is not injected is performed by subtracting a radiographic image in which a contrast agent is not injected from a radiographic image in which a specific portion of a subject (for example, a blood vessel when a human body is set as a subject) is enhanced by injection of a contrast agent. Using the so-called time subtraction to extract a specific part (for example, a blood vessel) and the fact that the specific part of the subject has different radiation absorptivity for radiation having different energies, the same subject can be compared with each other. By irradiating radiations having different energies, a plurality of radiation images of each radiation having different energies are obtained, and a specific portion of the subject is extracted by appropriately weighting the plurality of radiation images and calculating a difference therebetween. There is so-called energy subtraction. The present applicant has also proposed energy subtraction using a stimulable phosphor sheet (Japanese Unexamined Patent Publication No. 59-1984).
No. 83486, JP-A-60-225541).

(Problems to be Solved by the Invention) The image after the energy subtraction processing is obtained by subtracting a plurality of radiation images before the processing (hereinafter, the radiation image before the energy subtraction processing is referred to as an “original image”). However, since this is an image, the S / N ratio is lower than that of the original image, and the image becomes difficult to view.

For example, a subject composed of a soft part and a bone part such as a human chest is irradiated with radiation having different energies to obtain a plurality of radiation images, and the plurality of radiation images are read to represent each of the plurality of radiation images. Obtain a plurality of image data, perform energy subtraction processing based on the plurality of image data, and obtain soft image data representing a soft image in which mainly a soft part of the subject is recorded, or a bone image in which the subject mainly has a bone. In some cases, bone image data to be represented is obtained, and the obtained soft image or bone image is an observation target. Since the soft part image and the bone part image are images in which the shadow of the bone part and the soft part have been eliminated, respectively, the shadow hidden by the bone part or the soft part or the shadow that has become difficult to see due to the influence of the bone part or the soft part. May emerge and may well match a predetermined observation purpose. However, as described above, since the soft part image and the bone part image are images obtained by the subtraction processing, the noise component is emphasized as compared with the original image, and from this point the observation suitability is rather deteriorated.

The present invention has been made in view of the above circumstances, and has as its object to provide a method of generating a subtraction image having reduced noise to substantially the same level as an original image before the subtraction processing and having excellent observation suitability.

(Means for Solving the Problems) An energy subtraction image generation method according to the present invention is directed to a method for generating a plurality of radiations, which are transmitted from a subject composed of a plurality of tissues having different radiation absorptances and are obtained from radiations having different energy distributions. Based on a plurality of original image data representing each of the images, first image data representing a first image in which the first tissue in the subject is mainly recorded is obtained, and the first image data is processed. The first smoothed image data representing the first smoothed image from which the noise component of the first image has been reduced or removed is obtained, and the first smoothed image data is subtracted from the original image data. A second image data representing a second image in which a second tissue of the subject is mainly recorded. .

Here, when carrying out the above method, the method is substantially the same as the above method using various modified methods such as breaking down the method into more detailed steps and changing the order of the operations. The present invention can be understood as a concept including various methods that are substantially the same.

For example, although different in terms of expression, an example of a method substantially the same as the above-described present invention is a method in which a plurality of radiations having different energy distributions are transmitted through a subject composed of a plurality of tissues having different radiation absorption rates. Based on a plurality of original image data representing each of the radiation images, first image data representing a first image in which the first tissue in the subject is mainly recorded, and second image data representing the first tissue in the subject. And obtaining second image data representing a second image in which the tissue is recorded, processing the first image data, and the first tissue component mainly serving as a low spatial frequency component of the first image The noise image data representing the noise image from which the noise image has been reduced or removed is obtained, and the second image data and the noise image data are added to each other to obtain a noise image data. Aspects of finding a new second image data of the organization representing the new second image recorded can be mentioned.

This embodiment performs substantially the same calculation as the first energy subtraction image generating method of the present invention, as will be described in the embodiments described later. Is included.

In addition, the second energy subtraction image generation method of the present invention includes the steps of: transmitting a plurality of radiation images obtained from radiation having different energy distributions, which have passed through a subject composed of a plurality of tissues having different radiation absorption rates; After performing a first process of obtaining first image data representing a first image in which a first tissue in the subject is mainly recorded based on a plurality of original image data representing the first image, Processing the data to obtain first smoothed image data representing a first smoothed image in which the noise component of the first image has been reduced; and obtaining the first smoothed image data from the original image data. By performing a second process of obtaining second image data representing a second image in which mainly a second tissue of the subject is recorded. After processing, the second image data is processed to obtain second smoothed image data representing a second smoothed image in which a noise component of the second image has been reduced, and the original image data A third process of obtaining new first image data representing a new first image mainly recording the first tissue of the subject by subtracting the second smoothed image data from Is performed.

Here, the second processing and the third processing in the second energy subtraction image generation method are repeatedly performed, whereby it is possible to obtain an image having better image quality performance. That is, the third energy subtraction image generation method of the present invention, after performing each processing in the second energy subtraction image generation method, the new first image data obtained by the third processing By performing the second process again as the first image data in the second process,
A new second process for obtaining new second image data representing a new second image in which mainly the second tissue of the subject is recorded, and the new second image data By performing the third processing again as the second image data in the processing of the above, new first image data representing a new first image in which mainly the first tissue of the subject is recorded The new third process to be obtained is repeated once or a plurality of times.

Further, it is also possible to apply the second or third energy subtraction image generation method to finally obtain second image data representing a second image in which the second tissue of the subject is recorded. . That is, the fourth energy subtraction image generation method of the present invention is obtained by performing the processing in the second or third energy subtraction image generation method, and then performing the third processing or the new third processing. By performing the second process or the new second process again as the first image data in the second process or the new second process the new first image data, It is characterized in that new second image data representing a new second image mainly recording the second tissue of the subject is obtained.

Here, the second to fourth energy subtraction image generation methods include the same steps as the first energy subtraction image generation method, and thus the first energy subtraction image generation method has been described. Similarly, the above-described second to fourth energy subtraction image generation methods can be understood as concepts including substantially the same various aspects.
Further, the present invention is naturally included in the present invention as long as it includes each of the above methods including substantially the same method. For example, before carrying out the present invention, steps such as noise reduction processing by another method are included. Other steps may be included to further reduce noise after implementing the present invention.

The “first image” (including the “new first image”) and the “second image” (including the “new second image”) in each of the energy subtraction image generation methods. Are two images obtained by the energy subtraction processing, in which the shadows of different tissues of the same subject are emphasized or extracted, and are not limited to specific ones. For example, the above-described soft part image and bone part image And an image in which a mammary gland is enhanced and an image in which a malignant tumor is enhanced when a human breast is taken as a subject.

(Operation) The present invention has been achieved by noting that the image obtained by the energy subtraction processing has a reduced S / N ratio due to the execution of the subtraction processing (subtraction processing).

That is, the first energy subtraction image generation method according to the present invention provides a method for generating an energy subtraction image obtained by subtraction processing.
The first image of one of the two images (the first image and the second image) is obtained, and a first smoothed image in which a noise portion is reduced or removed is obtained from the image. Since the one smoothed image is subjected to the subtraction processing, the noise is reduced to the same degree as that of the original original image, and the second image having excellent observation suitability is generated.

Here, in order to obtain a high quality second image, it is necessary to delete only the noise component while maintaining the shadow of the first tissue of the subject when obtaining the first smoothed image. However, the shadow and noise components of the first tissue partially overlap with each other in spatial frequency. Therefore, even if a nonlinear filter that removes only the noise component as much as possible is used, the shadow and noise of the first tissue will There is naturally a limit to complete separation from components.

Therefore, the second to fourth energy subtraction image generation methods of the present invention reduce noise by abandoning complete separation of noise in one noise reduction process and repeatedly performing the noise reduction process. It is intended to generate an image with excellent observation suitability.

That is, the second energy subtraction image generation method of the present invention reduces the noise component by processing the first image data, and then obtains the second image data and processes the second image data. In this way, new first image data is obtained by further reducing the noise component. In the two noise reduction processes described above, it is possible to reduce the respective noise components, each of which is superior. An image with further reduced noise and more excellent observation suitability than the first energy subtraction image generation method is generated.

Further, the third energy subtraction image generation method of the present invention is intended to further reduce noise by repeating the above second energy subtraction image generation method. Process can be shared,
An image with further reduced noise is generated.

Further, the fourth energy subtraction image generation method of the present invention, after performing the second or third energy subtraction image generation method, a new energy subtraction image generation method obtained by the second or third energy subtraction image generation method Since the noise reduction processing is performed on the first image data to perform the subtraction processing from the original image, a new second image with reduced noise components is generated.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. Here, an example in which the above-described stimulable phosphor sheet is used will be described.

 FIG. 9 is a schematic diagram of an X-ray imaging apparatus.

X-rays 3 emitted from an X-ray tube 2 of the X-ray imaging apparatus 1
Illuminates the subject (the chest of the human body) 4. Subject 4
X-rays 3a transmitted through the first stimulable phosphor sheet 5 are irradiated to the first stimulable phosphor sheet 5, and X-rays having relatively low energy among the energies of the X-rays 3a are accumulated in the first stimulable phosphor sheet 5, Thus, an X-ray image of the subject 4 is accumulated and recorded on the sheet 5. The X-rays 3b that have passed through the sheet 5 further pass through a filter 6 that cuts low-energy X-rays, and the high-energy X-rays 3c that have passed through the filter 6 are applied to the second stimulable phosphor sheet 7. Thereby, the subject 4 is also placed on the sheet 7.
Are stored and recorded. The subject 4 is provided with two marks 8 that serve as references for aligning two X-ray images when performing the subtraction processing.

In addition, the above-mentioned X-ray imaging apparatus uses two sheets 5,
Although an X-ray image is stored and recorded in the storage unit 7, one image may be taken at each of two timings that are temporally successive.

FIG. 10 is a perspective view of an X-ray image reading device and an image processing and display device for implementing the energy subtraction image generating method of the present invention.

After radiography is performed by the X-ray radiographing apparatus 1 shown in FIG.
Each of the first and second stimulable phosphor sheets 5, 7 is X
The line image reading device 10 is set at a predetermined position. Here, the case of reading the first X-ray image stored and recorded on the first stimulable phosphor sheet 5 will be described.

The stimulable phosphor sheet 5 set at a predetermined position and on which the first X-ray image is stored is recorded by a sheet conveying means 15 such as an endless belt driven by a driving means (not shown).
Is conveyed (sub-scan) in the direction of arrow Y. On the other hand, a light beam 17 emitted from a laser light source 16 is reflected and deflected by a rotating polygon mirror 19 driven by a motor 18 and rotated at a high speed in the direction of the arrow Z, passes through a focusing lens 20 such as an fθ lens, and then passes through an optical path by a mirror 21. Stimulable phosphor sheet
14 and main-scans in an arrow X direction substantially perpendicular to the sub-scanning direction (arrow Y direction). From the portion of the stimulable phosphor sheet 14 where the light beam 17 was irradiated, the stored and recorded X
Stimulated light 22 is emitted in a quantity of light corresponding to the line image information. The stimulated light 22 is guided by a light guide 23 and is photoelectrically detected by a photomultiplier (photomultiplier tube) 24. The light guide 23 is formed by molding a light-guiding material such as an acrylic plate, and is arranged so that a linear incident end face 23a extends along a main scanning line on the stimulable phosphor sheet 14. The light receiving surface of the photomultiplier 24 is connected to the emission end surface 23b formed in a ring shape. Incident end face 23
The stimulated emission light 22 entering the light guide 23 from a repeats the total reflection inside the light guide 23, and proceeds to the exit end face 23b.
The photostimulable light 22 emitted from the photomultiplier 24 is received by the photomultiplier 24 and represents a radiation image.
Is converted into an electric signal.

The analog signal S output from the photomultiplier 24 is logarithmically amplified by a log amplifier 25, and then input to an A / D converter 26 where it is sampled to obtain a digital image signal SO. This image signal SO represents the first X-ray image stored and recorded on the first stimulable phosphor sheet 5, and is referred to as a _first image signal SO1 here. This first image signal SO ₁ is temporarily stored in an internal memory in the image processing display device 30.

The image processing and display apparatus 30 includes a keyboard 31 for inputting various instructions, a CRT display 32 for displaying auxiliary information for instructions and a visible image based on an image signal, and a floppy disk as an auxiliary storage medium loaded and driven. A floppy disk drive unit 33 and a main unit 34 having a CPU and an internal memory are provided.

Next, similarly to the above, the second stimulable phosphor sheet 7
Second image signal SO ₂ are obtained for representing the second X-ray image stored records, this second image signal SO ₂ is also temporarily stored in the internal memory of the image processing and displaying apparatus 30.

FIG. 1 shows two image signals representing first and second X-ray images stored in an internal memory in an image processing and display device.
FIG. 4 is a diagram showing an example of a flow of processing performed in the image processing display device based on SO ₁ and SO ₂ .

The first and second X-ray image signals SO ₁ and SO ₂ stored in the internal memory of the image processing and display device are respectively a first line image 41 and a second X-ray image 42 shown in FIG. Is a signal carrying. The first X-ray image 41 is an image based on relatively low energy X-rays, and the second X-ray image 42 is an image based on relatively high energy X-rays. Both are original images in which both the soft part and the bone part are recorded.

These first and second X-ray image signals SO ₁ , SO ₂ are
0 is read from the internal memory in the image processing and display device 30. First, the relative alignment of the X-ray images 41 and 42 carried by these two image signals SO ₁ and SO ₂ respectively is determined on the image signals. (See JP-A-58-163338).
This positioning is performed by relatively linearly moving and rotating the two X-ray images so that the two marks 8 shown in FIG. 1 overlap.

 Thereafter, a subtraction process is performed.

Here, the absorption coefficient μ of X-rays is determined as follows, separately for the subject and the bones of the soft part, and for low-energy X-rays and high-energy X-rays.

μL ^T : Absorption coefficient of soft part by low energy X-ray μH ^T : Absorption coefficient of soft part by high energy X-ray μL ^B : Absorption coefficient of bone by low energy X-ray μH ^B : Absorption coefficient of bone by high energy X-ray At this time, for each corresponding pixel of the _two image signals SO ₁ and SO ₂ , However, C performs weighted subtraction in accordance with the bias component to obtain a bone image signal S1 representing the bone image 43 (see FIG. 3) from which the shadow of the bone is extracted.

Also, the formula Where C 'is a soft image signal representing a soft image by performing weighted subtraction in accordance with the bias component.
Although S2 can be obtained, this operation is unnecessary in this embodiment.

Further, an addition process is performed for each pixel corresponding to each other in accordance with the equation SO = (SO ₁ + SO ₂ ) / 2 (3) to generate a superimposed image 44 of the two X-ray images 41 and 42. This superimposed image 44 is also an original image in which both the soft part and the bone part are recorded. Although the X-ray image 41 or the X-ray image 42 can be used instead of the superimposed image 44, the superimposed image 44 is composed of two X-ray images 41 and 42.
Are superimposed on each other, so that the noise component is reduced as compared with any of these X-ray images, which is advantageous for the subsequent processing.

Next, by processing the bone image signal S1, the bone image
The noise component included in 43 is extracted.

FIG. 2 is a diagram illustrating a spectrum with respect to a spatial frequency f of a bone part image and an image obtained by processing the bone part image signal.

The graph 51 shown in the figure represents the spectrum of the bone image 43, and includes a noise component 53.

Here, first, the bone image signal S1 is subjected to a smoothing process. As this smoothing processing method, for example, for each pixel, an average of image signals corresponding to each pixel in a predetermined area centered on the pixel is obtained, and this average value is used as a simple average as an image signal of the central pixel. A method using a median filter that sets the median value (median) of the image signal in the predetermined area as the image signal of the pixel at the center, and further divides the predetermined area into a plurality of small areas, Edge-preserving filter (V-filter) that obtains the variance and sets the average value of the small area having the smallest variance to the value of the image signal of the central pixel.
Can be used, a method of performing an inverse Fourier transform after removing the high spatial frequency component corresponding to the noise component by performing a Fourier transform of the image signal, and the like, but the above-described blur mask processing method has a disadvantage that the edge is blurred. Also, since the method using the median filter involves exchanging images, contour-like artifacts may occur.In addition, when the edge-preserving filter is used, honeycomb-like artifacts may occur. There is a problem that the calculation takes time.
Therefore, in this embodiment, smoothing is performed using the following histogram adaptive filter which is different from any of the above methods. By using this method, it is possible to remove noise without preserving the necessary edges (step-like density changes that define the boundaries of two different tissue shadows) as image information and eliminate the artifacts. And has the advantage that noise can be removed in a short time.

First, for each pixel of the bone image, a histogram of the image signal S1 of a large number of pixels in a predetermined area centered on the pixel is created.

FIGS. 3A and 3B are plots of the appearance frequencies of the image signal S1 corresponding to a large number of pixels within a predetermined area centered on a certain pixel (image signal S1 ′) obtained as described above. FIG. 4 is a diagram showing two different histograms. FIG. 4 is a diagram showing an example of a function using a difference between the image signal S1 and the image signal S1 'of the central pixel as a variable.

A function representing a histogram as shown in FIGS. 3A and 3B is generally represented by h (S1), and a function which monotonically decreases as the absolute value | S1-S1 '| Let f (S1−S1 ′). At this time, a function g (S1) representing the frequency after processing is obtained in accordance with the equation g (S1) = h (S1) × f (S1-S1 ′) (4).
When the function h (S1) has a plurality of peaks as shown in FIG. 3A, the function g (S1) has an effect of extracting only the peak to which the image signal S1 'of the central pixel belongs.

After calculating the function g (S1) according to the above equation (4), the average value of the image signal S1 weighted by the function g (S1) is obtained.
Ask for ▼. That is, specifically, for example, the function g (S1)
Is calculated according to the following equation.

▲ ▼ = ∫g (S1) × S1dS1 / ∫S1dS1 (5) The processing according to the above equations (4) and (5) is performed with each pixel of the bone image as the center pixel, whereby the smoothed image signal is obtained. ▲ ▼ (for simplicity, the same signal is used for the image signal corresponding to each pixel and the image signal representing the whole image). This smoothed image signal ▲
▼ is a signal obtained by removing the high spatial frequency component of the original bone image signal S1 as shown in the graph 52 of FIG. 2, and the pixel near the edge is the pixel as shown in FIG. 3A. Since the signal is obtained by calculating the average value after extracting only the mountain to which the mountain belongs, the edge in the original bone image is stored without blurring.

Next, from the superimposed image signal SO (see equation (3)) representing the superimposed image 44 for each pixel, the smoothed image signal
▼ weighted subtraction, that is, However, C ″ represents a bias component.
As the image information, the processed soft part image 46 (FIG. 1) which carries substantially the same information as the soft part image represented by the above equation (2) and has a reduced noise component than the soft part image represented by the above equation (2) See).

The image signal S2 'obtained according to the equation (6) is sent to the CRT display 32 of the image processing and display device 30, and a visible image based on the image signal S2' is reproduced and displayed on the CRT display 32.

In the above embodiment, the soft image signal S2 'is obtained by smoothing the bone image signal S1 and subtracting the soft image signal S2' from the original image. Then, the soft part image signal S2 is obtained, and the soft part image signal S2 may be smoothed and subtracted from the original image to obtain a bone part image with a reduced noise component.

Next, the processing is substantially the same as that of the above embodiment described with reference to FIG. 1, and therefore, another embodiment included in the present invention will be described.

FIG. 5 illustrates this substantially identical embodiment,
Based on _two image signals SO ₁ and SO ₂ representing the first and second X-ray images stored in the internal memory in the image processing and display device, the flow of processing performed in the image processing and display device is described. FIG. 3 is a diagram showing an example of the above. The same elements in FIG. 1 are denoted by the same reference numerals and symbols as those in FIG. 1, and the description of the portions described with reference to FIG. 1 is omitted here to avoid redundant description.

From the two X-ray images 41 and 42, a bone image 43 (bone image signal S1) and a soft image 47 based on the above equations (1) and (2).
(Soft part image signal S2) is obtained.

Next, by processing the bone image signal S1 based on the above equations (4) and (5) in the same manner as in the above-described embodiment,
A smoothed image signal ▲ ▼ in which the noise component included in the bone image 43 is reduced is obtained, and thereafter, the smoothed image signal ▲ ▼ is subtracted from the bone image signal S1 for each pixel, so that only the noise component is obtained. The noise image 48 (noise signal S _N ) from which is extracted is obtained.

S _N = S1− ▲ ▼ (7) The noise signal S _N is a signal obtained by extracting a noise component of the bone image as shown in a graph 53 of FIG. Here, since the information of the edge of the bone image of the smoothed image signal 信号 is stored even if the spatial frequency is as high as the noise component, the smoothed image signal S1 is smoothed with the bone image signal S1 according to the above equation (7). By obtaining the difference from the image signal ▲ ▼, the edge information is clearly canceled, and therefore, the noise signal _SN is more purely compared with the case where the smoothing process for losing the edge information is performed. Is a signal carrying only the noise component of.

Next, the noise signal S _N obtained in this way and the soft image signal S2 representing the soft image 47 (see FIG. 5) are weighted and added for each pixel, whereby image information is obtained as the soft image 47. A processed soft part image 46 that carries substantially the same information as that of the soft part image 47 and has a noise component reduced compared to the soft part image 47.
Is required. In the present embodiment, this weighted addition is represented by an equation , Whereby noise components are further reduced.

Here, it will be described that the embodiment described with reference to FIG. 1 and the embodiment described with reference to FIG. 5 are substantially the same.

The soft part image signal represented by the above equation (2) in the above equation (8)
S2 and the noise signal _SN shown in the above equation (7) are substituted. It should be noted that the bias component (eg, C ′ in the above formula (2)) adjusts the finally obtained density of the entire image (including the luminance when displayed on a CRT display device or the like). Will be omitted.

By substituting equations (2) and (6) into equation (8), By substituting the bone image signal S1 expressed by the above equation (1) into the equation (9) (ignoring the bias C), By rearranging this equation (10), And further substituting the above equation (3), Becomes This equation (12) is the same as the above equation (6) except for the bias component. That is, substantially the same processing is performed in the embodiment described with reference to FIG. 1 and the embodiment described with reference to FIG.

FIG. 6 is a diagram showing a flow of processing according to another embodiment of the present invention, and FIG. 7 is a diagram schematically showing a profile of each image shown in FIG. 6 in a predetermined direction. .

6, elements corresponding to those in FIG. 1 or FIG. 5 are denoted by the same reference numerals and symbols as those in FIG. 1 and FIG. 5, and redundant description is omitted.

FIGS. 7A and 7B are X-ray images (original images), respectively.
FIG. 4 is a diagram schematically showing 41 and 42, in which values of image signals SO ₁ and SO ₂ along a predetermined one direction (x direction) on X-ray images 41 and 42 are plotted. Image signals SO ₁ , SO ₂
Are superimposed with a signal component representing a uniform soft part (shaded portion in the figure) but a signal component representing a bone part changed in a step-like manner, and a random noise component having different values from each other. ing.

A soft part image signal is obtained by performing a weighted subtraction process (represented by a symbol) based on the above equation (2) based on these two image signals SO ₁ and SO ₂ representing two X-ray images (original images) 41 and 42. A soft image signal S2 representing 47 is obtained, and an addition process (represented by a symbol) is performed on the basis of the above equation (3) based on the _two image signals SO ₁ and SO ₂ , thereby obtaining a superimposed image 44 representing the superimposed image 44 The signal SO is required.

FIG. 7 (c) is a diagram schematically showing the superimposed image signal SO. Similar to FIGS. 7 (a) and 7 (b), a uniform signal component representing a soft part (the hatched portion in FIG. 7), a signal component representing a bone part changed stepwise, and a random noise component are superimposed. This noise component is composed of two X-rays shown in FIGS. 7 (a) and 7 (b). Images 41 and 42
It is reduced compared to.

FIG. 7D is a diagram showing the soft part image signal S2 obtained based on the above equation (2). Although only signal components representing uniform soft parts are extracted, random noise components are extracted from the two X-ray images 41 and 42 (FIGS. 7A and 7A).
(B) More than any of the above.

Further, although it is not necessary to obtain the bone image signal S1 in the present embodiment, it is a diagram showing the bone image signal S1 when the bone image signal S1 is obtained based on the above equation (1). A signal component representing a bone part changed in a step-like manner is extracted. Like the soft part image signal S2 (FIG. 7 (d)), a random noise component is generated from the two X-ray images 41 and 42 (the 7 (a),
(B) More than any of the above.

Here, the soft part image 47 (soft part image signal S2, smoothing processing 51 (see FIG. 6) is performed on FIG.
A smoothed soft part image signal ▲ (FIG. 7 (f)) representing 61 is obtained. In the smoothing process 51, a high spatial frequency component of, for example, 1.0 cycle / mm or more of the soft part image 47 is cut.

Next, the smoothed soft part image signal ▲ is obtained from the superimposed image signal SO.
▼ is weighted and subtracted, whereby a bone image signal S1 ′ representing the bone image 62 is obtained. This bone image signal
S1 'is a bone image signal S1 as shown in FIG.
Although the random noise component is reduced as compared with (FIG. 7 (e)), the effect of smoothing the soft part image 47 appears, and the high spatial frequency component of the soft part image is slightly mixed.

Next, a smoothing process 52 is performed on the bone image signal S1 'obtained as described above. Smoothing processing 52 performed here
In the example, the low-contrast shadow (bone image signal S1 ′) in the spatial frequency band of 0.5 cycle / mm or more of the bone image 62, for example.
) Is cut off. As the treatment method, for example, 0.5 cycles for a given pixel P _O /
Consider the window area corresponding to mm, this' among the signal S1 _O corresponding to a predetermined pixel P _O 'each signal S1 corresponding to the respective pixels in the window signal S1 is a value ± a predetermined value of' Of the predetermined pixel P _O
Bone image 62 using a filter to be a new signal S1 _O ′
A method of scanning the upper side or the like is adopted. By this smoothing process 52, the smoothed bone image signal ▲ representing the smoothed bone image 63
▼ 'is required. This smooth bone image signal ▲
In FIG. 7 (i), as shown in FIG. 7 (i), although the noise component and the high-frequency component of the mixed soft part image are reduced, the rising part is also dull.

Next, the smoothed bone image signal ▲
▼ ′ is weighted and subtracted to obtain a southern image signal S2 ′ representing the soft part image 64. As shown in FIG. 7 (h), the soft part image 64 has a reduced noise component than the soft part image 47 (FIG. 7 (d)), but has a smooth bone image signal ▼ ′ ′ (7 Since the rising part in FIG. (I) is dull, the information of the bone image of that part is superimposed as noise. However, the information of the random noise portion and the bone image as noise is considerably small. Therefore, a series of processing is stopped at this stage, and the soft image signal S2 'is displayed on the CRT display 32 of the image processing display device 30 (see FIG. 10). And a visible image based on this soft image signal S2 '
The information may be reproduced and displayed on the display for observation.

However, in the present embodiment, the same processing as described above is further repeated to further improve the image quality.

After obtaining the soft part image signal S2 'representing the soft part image 64, the soft part image signal S2' is subjected to a smoothing process 53, and the smoothed soft part image signal ▲ ▼ 'representing the smoothed soft part image 65 (FIG. 7 ( j)) is required. As the smoothing process 53, for example, a process of cutting a spatial frequency component of 1.5 cycles / mm or more is performed.

The smoothed soft part image signal ▼ ′ is subjected to a weighted subtraction process from the superimposed image signal SO to obtain a bone part image signal S1 ″ representing the bone part image 66. This bone part image 66 is shown in FIG. ), The bone image 62 (FIG. 7 (g))
In comparison with this, the random noise and the information of the soft part image mixed as noise are also reduced. When a bone image is to be observed, a visible image based on the bone image signal S1 ″ may be reproduced and displayed on the CRT display 32.

In the present embodiment, a smoothing process 54 is further performed on the bone image signal S1 ″ obtained as described above,
A smoothed bone image signal ▼ ″ (FIG. 7 (m)) representing 67 is obtained.As the smoothing process 54, for example, a low-contrast component of 1.0 cycle / mm or more is cut.

Next, the smoothed bone image signal ▲ ▼ ″ is weighted and subtracted from the superimposed image signal SO to obtain a soft image signal S2 ″.
Is required. As shown in FIG. 7 (l), this soft part image signal S2 ″ is different from the previously obtained soft part image signal S2 ′ (FIG. 7 (h)) in both the random noise and the information of the bone image as noise. In both cases, the signal is further reduced.

By repeating the smoothing process and the weighted subtraction of the superimposed image (original image) in this manner, it is possible to alternately obtain a bone image and a soft image in which noise is sequentially reduced.

FIG. 8 is a diagram showing the flow of another process substantially the same as the embodiment described with reference to FIG. 6 and the like are denoted by the same reference numerals and symbols as in FIG. 6 and the like, and description thereof is omitted.

The processing shown in FIG. 8 is the processing until the bone image 62 shown in FIG. 6 is obtained (the processing described with reference to FIG. 1 (however, the bone image and the soft part image are replaced with those in FIG. 1). Is)),
The processing described with reference to FIG. 2 is replaced with the processing described with reference to FIG. 2 (however, the bone image and the soft part image are replaced with each other in FIG. 2). is there.

In the processing shown in FIG. 8, only the first stage is replaced by the processing method described with reference to FIG. 2. However, this replacement can be performed at an arbitrary stage of the repeatedly performed processing, and in any case, The processing is the same, and the present invention includes all substantially the same processing modes changed in any one or more of these steps.

Each of the above embodiments is an example in which a soft image or a bone image is obtained based on an X-ray image of the chest of a human body.
Further, the present invention is not limited to an image for obtaining a soft part image or a bone part image. For example, an image in which a mammary gland is emphasized or an image in which a malignant tumor is emphasized may be used. The present invention can be widely applied when obtaining one or both of two images which are respectively emphasized or extracted.

Further, the above embodiment is an example in which a stimulable phosphor sheet is used, but the present invention is not limited to a stimulable phosphor sheet, but an X-ray film (generally combined with an intensifying screen when photographing). And the like can be applied.

(Effects of the Invention) As described in detail above, the energy subtraction image generation method of the present invention obtains first image data representing a first image of a subject mainly recording a first tissue, and obtains the first image data. The first smoothed image data is obtained by reducing or removing the noise component of the first image data, and the second smoothed image data is obtained by subtracting the first smoothed image data from the original image data. As a result, an image with reduced noise components and excellent observation suitability is generated.

Also, the first image and the second image are alternately smoothed,
By repeating the subtraction process from the original image, a first image and a second image in which noise components are further reduced can be generated.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a flow of a process performed in the image processing display device. FIG. 2 is a diagram showing a spatial frequency spectrum of a bone image and an image obtained by processing the bone image signal. FIGS. 3A and 3B are diagrams showing two different histograms in which the appearance frequencies of image signals corresponding to a large number of pixels in a predetermined area centered on a certain pixel are plotted. FIG. FIG. 5 shows an example of a function using the difference between the image signal S1 and the image signal S1 'of the pixel at the center of the predetermined area as a variable. FIG. 5 shows FIG. FIG. 6 is a diagram showing a flow of another process substantially the same as the process shown in FIG. 6, FIG. 6 is a diagram showing a flow of a process according to another embodiment of the present invention, and FIG. FIG. 8 is a diagram schematically showing a profile in a predetermined direction, FIG. FIG. 9 is a schematic diagram of an X-ray imaging apparatus, and FIG. 10 is a diagram showing an X-ray image reading apparatus and an energy subtraction image generation method of the present invention. It is a perspective view of the implemented image processing display. DESCRIPTION OF SYMBOLS 1 ... X-ray imaging apparatus, 2 ... X-ray tube 3, 3a, 3b, 3c ... X-ray, 4 ... Subject 5 ... First stimulable phosphor sheet 6 ... Filter 7 ... Second Storage phosphor sheet 8 Mark 16 Laser light source 19 Rotating polygon mirror 22 Stimulated emission light 23 Light guide 24 Photomultiplier 25 Log amplifier 26 A / D converter 30 Image processing display device 41,42 X-ray image (original image) 43,62,66 Bone image 44 Image overlay (original image) 45,63,67 … Smoothed bone images 46,47,64… Soft images, 48… Noise images 61,65… Smoothed soft images 51,52,53,54… Smoothing processing

Claims (4)

(57) [Claims] 1. A method according to claim 1, further comprising: transmitting a plurality of radiation images having different energy absorptivity to each other; Obtaining first image data representing a first image in which a first tissue is mainly recorded in the subject, and reducing or removing a noise component of the first image by processing the first image data. First smoothed image data representing the first smoothed image obtained is obtained, a superimposed image is obtained by superimposing the plurality of original image data, and the first smoothing is performed from the superimposed image. A second image data representing a second image in which a second tissue of the subject is mainly recorded by subtracting the image data. Guy subtraction image generation method. 2. Based on a plurality of original image data representing respective ones of a plurality of radiation images obtained from radiations having mutually different energy distributions and transmitted through a subject constituted by a plurality of tissues having different radiation absorptances from each other. After performing a first process for obtaining first image data representing a first image in which a first tissue is mainly recorded in the subject, the first image data is processed to process the first image data. By obtaining first smoothed image data representing a first smoothed image in which the noise component of the image has been reduced, and subtracting the first smoothed image data from the original image data, By performing second processing for obtaining second image data representing a second image in which the second tissue is mainly recorded, and processing the second image data after the second processing. By obtaining second smoothed image data representing a second smoothed image in which the noise component of the second image has been reduced, by subtracting the second smoothed image data from the original image data And performing a third process of obtaining new first image data representing a new first image in which the first tissue of the subject is mainly recorded. 3. After performing the processing according to claim 2, the new first image data obtained by the third processing is used again as the first image data in the second processing. By performing the second process, a new second process for obtaining new second image data representing a new second image mainly recording the second tissue of the subject, and the new second process By performing the third process again as the second image data in the third process using the second image data, a second image mainly representing the first tissue of the subject is recorded. A method for generating an energy subtraction image, comprising repeating one or more times a new third process for obtaining new first image data. 4. After performing the processing according to claim 2 or 3, the new first image data obtained by the third processing or the new third processing is subjected to the second processing or By performing the second processing or the new second processing again as the first image data in the new second processing, a new second processing in which mainly the second tissue of the subject is recorded An energy subtraction image generation method, wherein new second image data representing a second image is obtained.